cve-check: Only include installed packages for rootfs manifest

Before this the rootfs manifest and the summary were identical.
We should separate the summary and rootfs manifest more clearly,
now the summary is for all CVEs and the rootfs manifest is only for
things in that image. This is even more useful if you build multiple
images.

Signed-off-by: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 3b8cc6fc)
Signed-off-by: Steve Sakoman <steve@sakoman.com>